Like this. http://en.wikipedia.org/wiki/United_Nations_Convention_Against_Torture
Any act by which severe pain or suffering, whether physical or mental, is intentionally inflicted on a person for such purposes as obtaining from him or a third person, information or a confession, punishing him for an act he or a third person has committed or is suspected of having committed, or intimidating or coercing him or a third person, or for any reason based on discrimination of any kind, when such pain or suffering is inflicted by or at the instigation of or with the consent or acquiescence of a public official or other person acting in an official capacity. It does not include pain or suffering arising only from, inherent in or incidental to lawful sanctions.
Check out who signed this Convention in 1988, and ratified it in 1994.
By the way, lawful sanction means incarcerated into a cell like other prisoners. No court would have issued as punishment for a crime the practice of waterboarding.
Just because the Government says it's perfectly acceptable, doesn't mean it's perfectly acceptable.
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